The ability to control polarization properties of optical radiation is of importance for many applications. In particular, pumping of optical amplifiers, such as erbium-doped fiber amplifiers (EDFA) and Raman amplifiers, which are commonly employed in optical communication systems, typically require sources of high-power laser radiation having a very low degree of polarization (DoP).
The DoP of optical radiation is commonly defined as a ratio of the optical power of a prevailing polarization component of the optical radiation to the total optical power thereof, with a DoP equal to zero corresponding to totally non-polarized light. In optical amplifiers, pump radiation having a non-zero DoP induces a polarization dependant gain (PDG), which is generally detrimental to the system performance. For Raman amplifiers, a low PDG can be achieved by using pump laser light with a low degree of polarization. However, high-power laser diodes commonly used for pumping of optical amplifiers emit strongly polarized optical radiation having a high degree of polarization.
A prior art solution to this problem is shown in FIG. 1, wherein orthogonally-polarized light from two pump lasers 120 and 130 is combined into a combined light beam 195 by using a polarization maintaining beam combiner (PBC) 190. If the optical radiation from individual pumps have equal DoPs at the optical input ports 191 and 192 of the PBC 190, and the polarization beam combiner has the same optical loss for radiation from both pump lasers, the combined laser beam 195 outputted from the PBC 190 is substantially depolarized, i.e. has DoP which is a close to zero, when the laser diodes have equal output powers.
In order to maintain equal pump powers from the pump lasers after the PBC 190 and therefore to maintain the DoP of the combined pump beam 195 at a low enough level, two tap couplers 15 are used in the path of radiation from each laser 120 and 130, with a small portion of the pump light being tapped to photodiodes 140 and 180 respectively. Electrical signals from the photodiodes 140, 180 are fed to a pump controller 110, which controls the output pump power from the pump lasers 120 and 130.
However, the “through” paths of the tap couplers 15, i.e. the optical paths from the output of the laser pumps 120 and 130 to the input ports 191 and 192 of the PBC 190 respectively, have to maintain the polarization of the lasers' radiation, to ensure that there is no change of the polarization of the pump radiation at the input PBC ports 191, 192 at different operating conditions. Therefore in the prior art configuration shown in FIG. 1, the taps 140 and 180 have to be preferably polarization maintaining, which are more expensive than conventional, non-polarization maintaining tap couplers.
Moreover, several pump laser pairs are often used in a single Raman pump module, since optical pumping at several pump wavelengths is required for reducing a Raman gain ripple, i.e. for reducing variations of the Raman gain for data channels at different wavelengths. The prior art pump control scheme, shown in FIG. 1, in that case becomes progressively more expensive, as the number of required polarization-maintaining taps and photodetectors increases proportionally with the number of pump lasers in the module.
It is therefore desirable to have a solution wherein the DoP of the combined radiation is controlled using a feedback signal extracted from the combined radiation after the last pump combiner, without tapping off optical power of each individual pump. The present invention provides a method and system of pump control accomplishing this task by employing distinctive pump modulation as pump “markers” that can be identified in the combined radiation and used to control the DoP thereof.
Various Raman pump modulation schemes have been disclosed in the prior art. For example, U.S. Pat. No. 6,597,495 issued to Gertsvolf et al. discloses synchronously modulating Raman pump lasers for suppressing four-wave mixing effects in the transmission fiber. U.S. Pat. No. 6,850,360 issued to Chen et al. discloses Raman pump modulation for fiber span characterization using optical time-domain reflectometry. U.S. Pat. No. 6,456,426 issued to Bolshtyansky et al. discloses Raman pump modulation for reducing cross-pump interactions in the transmission fiber.
U.S. patent application 2003/0095745 by Gehlot discloses low-amplitude RF modulation of the pump radiation for reducing pattern-dependent cross talk between WDM communication channels.
U.S. Pat. No. 6,731,428 issued to Gehlot discloses superimposing unique signature signals onto the RF-modulated Raman pump lasers for monitoring the performance of the fiber Raman amplifier system. The “signature” signals are extracted at the receiver after propagation through an optical fiber link. If a particular signature signal is noticeably weaker than other received signature signals, as evidenced by a low SNR or high BER, this is indicative of a power loss in its associated pump source.
U.S. patent application 2002/0094158 by Evans et al. discloses an optical fiber amplifier comprising at least one Raman pump modulated with an RF signal for pump power monitoring and Raman gain control of the amplifier. The Raman pump receives both a DC electrical input and an AC electrical input, and provides an optical pump power having both a DC optical power component Pdc and an AC optical power component m·cos(ωt). The optical fiber amplifier system also includes an optical pump power detector coupled to the pump, and at least one controller operatively connected to the pump power detector to determine the DC optical power component of the optical pump power.
The controller disclosed by Evans et al. detects amplitudes of the AC components of the photocurrent and the photocurrent squared at the frequency of pump modulation, which are proportional to m and Pdc·m respectively, and feeds them to a divider circuit to obtain a ratio of the detected amplitudes which is proportional to the DC optical power component Pdc. The controller then adjusts the DC electrical input to the pump based on the DC optical power component to affect the Raman gain of the amplifier Pdc.
Operation of this controller is described in U.S. patent application 2002/0094158 in reference to a single-pump system; however, it would be difficult to adopt the controller disclosed therein for an amplifier system, wherein pump power of multiple Raman pumps is combined to enable detection of the DC components of each individual pump. Although Gehlot shows embodiments wherein the aforedescribed controller is employed in multi-pump amplifiers, it is the Applicant's assertion that the controller described by Evans in U.S. patent application 2002/0094158 would provide a control signal proportional to a DC component of the combined radiation, rather than separating it into the DC components of the optical power of individual Raman pumps, wherefrom the combined radiation is formed.
Although the aforementioned inventions disclose various schemes of Raman pump modulation, none of them provide a solution for DoP control of the combined pump radiation.
It is therefore an object of the present invention to provide a pump combining laser system for pumping of optical amplifiers with a combined radiation from two or more pump lasers, wherein a degree of polarization of the combined radiation is maintained at a near-zero level by adjusting individual pump laser powers using a feedback signal extracted from the combined radiation.
It is another object of the present invention to provide a laser apparatus wherein orthogonally-polarized radiation of two distinctively modulated lasers is combined into combined radiation, and wherein the DoP of the combined radiation is controlled by adjusting a power ratio of the individual lasers based on a modulation detected in the combined radiation.
Another object of the present invention is to provide a method for monitoring and controlling the DoP of combined radiation from two laser sources of orthogonally-polarized radiation by adjusting their power ratio while monitoring a modulation signal in the combined radiation.